The sequence of events associated with the development and maturation of the chick trigeminal ganglion can be summarized as follows: 1) dual cellular origin by 3.5 days of incubation from neural crest and placodal ectoderm material; 2) a transitory topographical arrangement of small, medially located cells and of large, laterally located cells within the 5 to 10 day incubation ganglion; 3) an apparently random distribution of small and large neurons between 11 and 17 days of incubation; and 4) a random arrangement of dark and light neurons within 18 day incubation, post-hatching, and adult ganglia. Although it has been demonstrated that the neural crest cells were the precursors of the small cells and that the placodal ectoderm cells were the precursors of the large cells, no such correlation has been investigated concerning the dark and light appearing trigeminal neurons. It is the specific aim of this proposal to investigate the relationship between the topographically arranged small and large cells within early developing chick trigeminal ganglia, and the dark and light neurons observed within post-hatching and adult ganglia. Due to differences in nuclear morphology, the quail cell is quite easily distinguished from the chicken cell. Because of this fact and the great similarity between the two species, isotopic and isochronic grafts of both quail neural crest material and placodal ectoderm material will be orthotopically placed into chick embryos of the same age. In this manner, the migration of the quail cells and their fate in relation to small and large cells and dark and light cells can be investigated.